A dc rotating electrical machine typically includes a rotor surrounded by a wound stator. A rotor connected commutator with copper segments and stationary brushgear are used to control the commutation of current in the rotor winding based on the angular position of the rotor. Brush commutated dc electrical machines are known to be capable of high air gap shear stress but their practical torque density performance is limited by the brush commutation process. The stationary brushgear and rotating commutator converts the dc terminal voltage of the electrical machine into a polyphase ac voltage that rotates within the armature winding in a direction and at a speed that causes the armature magneto-motive force (mmf) to be substantially stationary and aligned in quadrature with the field poles. Thus, rotor mmf and stator field space harmonic spectra are substantially synchronised, thereby contributing to the mean shaft torque. It is this near ideal relationship between rotor and stator space harmonics that allows the majority of the air gap circumference of the brush commutated dc electrical machine to operate at a high air gap shear stress.
However, the brush commutation process for dc electrical machines is complicated and has certain limitations.
Armature coil voltage is used to cause armature current commutation. This means that the brush position must be set to allow sufficient time for current to be reduced to a low level by the time an outgoing commutator segment breaks contact with the trailing edge of the brush in order to avoid arcing (so-called ‘under-commutation’), and also to avoid excessive time for current reduction and subsequent current reversal by the time an outgoing commutator segment breaks contact with the trailing edge of the brush in order to avoid arcing (so-called ‘over-commutation’). Both under- and over-commutation arcing modes are typically destructive.
There is an overriding requirement that the voltage between commutator segments immediately after the outgoing commutator segment breaks contact with the trailing edge of the brush must be sufficiently low to avoid flashover. There is also an overriding requirement that the brush current density must be low in order to avoid excessive heating, power losses, and the risks of sustained arcing flashover.
In the most basic brush commutated dc electrical machine the timing of brush commutation is critical and is severely performance limiting because ideal brush angular position varies with both armature current and speed, i.e. there is no single ideal angular position for brushgear. It is therefore accepted that in such dc electrical machines some commutator arcing is inevitable. However, in large dc electrical machines the risks of arcing and flashover can be alleviated by the use of compoles (or interpoles) which serve to offset the field position in response to variation in armature current.
As a result, the rotating commutator and stationary brushgear tend to be large and complex. Moreover, the compoles occupy space within the electrical machine that could otherwise be used to increase the total air gap flux and torque density. This means that the torque density for a given peak air gap shear stress is relatively low. Brush commutated dc electrical machines are inherently low voltage machines, e.g. less than 1 kV.
Some of the problems of brush commutation can be overcome by the use of a load commutated inverter (LCI). In electrical machines that use a LCI the field is produced by the rotor winding which commonly incorporates brushless excitation. The armature winding is located in the stator and commonly uses three or six phases. A static frequency converter replaces the brush commutator. A basic naturally-commutated power converter operating at low switching frequency can be remotely located. Such electrical machines have increased torque density and high efficiency but are known to produce undesirable torque pulsations. They also cannot employ the near ideal relationship between rotor and stator space harmonics described above. Hence the mean air gap shear stress is typically less than that for a brush commutated dc electrical machine. However, one advantage of electrical machines that use an LCI is that it is possible to have a higher line voltage rating, e.g. up to 11 kV.
More sophisticated static frequency converters have been used to allow the torque pulsations of the LCI to be substantially eliminated but the converters are complex and are less efficient. As line voltage rating increases, such converters become increasingly complex and it is exceptional for them to be rated at greater than 6.6 kV.
Electronically commutated brushless dc electrical machines are known. GB 2117580 discloses a brushless dc electrical machine that employs an electronic switching circuit which uses armature coil voltage to cause natural commutation of thyristors. Other brushless dc electrical machines use auxiliary power circuits such as those disclosed in GB 2431528 to cause forced commutation by thyristor reverse recovery. These electronic commutators have been surpassed by the use of semiconductor power devices that are capable of being turned on and off by gate control, e.g. gate turn off thyristors (GTOs). Such electronic commutators are described in EP 1798847 to the present applicant. A possible shortcoming of electronically commutated electrical machines is that they are not inherently suited to high voltage dc operation since it is necessary to use series-connected semiconductor power devices and to insulate the main wall of the armature winding for high voltage ac stresses—note that the dominant voltage stress in the armature insulation is ac since each terminal in the armature winding is sequentially connected to positive and negative dc terminals.
EP 2403111 describes a wind turbine generator with a rotor and a stator. The stator has a plurality of stator coils, each coil being connected to a diode rectifier. A generator-utility grid interface is provided where the diode rectifiers are assigned to each phase of a utility grid.